ViribusUnits

Sterling, how would disconnecting the hall sensor retard the timing? I thought it would stop the ECU from getting the speed/position signal, and then the car won't run, I guess that would greatly reduce NOx, but I'm not so sure that is desirable. I'm far from an expert on the LH-stuff. I do understand that disconnecting the anti-knock sensors would cause the computer to read a failed sensor, and then the timing is retarded to avoid damage to the engine.

Dr. Bob, when you say that the HC is a function of a lean miss fire, and only a function of mixture if it's lean enough to cause a miss fire you are not right. It is ALSO a strong function of the air fuel mixture on the rich side. If the mixture is rich enough so that there is excess gasoline there is not enough air to burn the fuel, and the hydrocarbons will pass through unburned. This is what I was saying. I'm agreeing that a lean miss fire can cause the problem, but SO can a rich mixture.

Dr. Bob, I also realized that my paragraph wasn't exactly clear as to what I was trying to say. The sooner the spark plug fires, the lower the chamber temps and pressures are at the beginning of the burning process. However, as the piston continues upward and the burning continues, the pressures rapidly increase. This means that at the beginning of the burning process, the pressures are relatively low, in comparison. At the end of the burning process, the pressures are relatively HUGE, compared to a normal process timed correctly. I'm trying to place particular emphasis on how far the flame front has advanced through the chamber. When it just gets started, pressures and temps are low, when it finishes pressures and temps are relatively high. The more advanced the timing, the lower the beginning is, and the higher the ending is.

If the timing is too far advanced, the pressures and temps at the END of the burning process become so high, the reaction rate of the fuel reaches nearly instant, and you get detonation.

Detonation isn't the only thing that can cause NOx. For NOx to form, you need two things in the combustion chamber, "excess" O2, and high combustion chamber temps. Note, the combustion chamber temps are the highest for any given timing setting, when the air/fuel mixture is at stoch. However, NOx formation is highest just lean of stoch, so as you can see, you need excess O2.

Also, note the higher the combustion chamber temps, the more readily NOx is formed. (ie, the less excess O2 is needed for the reaction to happen.)

Lets say your running 106 octane fuel in your 928, but your 10degrees advanced off of your base line. Odds are your not going to get any detonation; the 106 octane fuel just won't let it happen. However, it's also likely that you’re going to get excessive NOx production. Just simple advanced timing, w/o detonation can cause a NOx issue, and you don't even need to run that much advance or weird gasoline.

Realist D.

ROG100,

This thread is pretty interesting. I'm sure everyone here will be waiting to read your report of a successful test. Given everything you've tried so far, is it too obvious to suggest that you try a different testing facility? There doesn't seem to be any real reason for the high NOX readings you continue to get and you have changed some things that should have changed your results but didn't. I sometimes suspect that these emission test facilities are a complete scam - they just dial in what results they want before they hook you up for a test.

ROG100

Realist D,
Thanks for the suggestion - been there done that. The second test was conducted by Porsche and came up with the same readings.
The third test which I did yesterday was yet another testing station and came up with similar readings.

Exotic MotorWerks in Phoenix had a look at the results and reckon that the engine is running lean and needs a new MAF.

Old & New

-Autobahn in Ft Worth- vs. -Exotic MotorWerks in Phoenix-

ROG100

Autobahn said it definitly was the cat so my money is on Exotic motorwerks.

If it is not the MAF and the vacuum is OK I am at a loss of where to go next.

My money is on ---------- its all my money!!!!!!

ViribusUnits

Where you able to check the ECU part numbers? And for that matter, the chips sets?

ROG100

Hi VibrusUnits,
Yes I did sorry forgot to update that info. They were the right numbers according to my PET. The screws all had a blue dye on them and had not been touched until I messed around. There were no scratchs to or tell tale marks to suggest somebody had been inside to mess about.
They looked like they had never been removed before.

Thanks for your help.

ViribusUnits

Well, just to double check and make sure it's fuctioning properly, pull out the trusty timeing light and check it.

wildguy

I do know that water injection has worked on cars that have had trouble passing. If the MAF doesn't solve the problem I would give it a shot. Also, do not go to the smog station on a muggy day.

Guy

Greggles

Having had a similar problem w/ Nox not too long ago what I found was the rpm speed (or gear used for the test) was the difference between passing & failing. In Va. they take two measurements one at 15mph & another at 25mph on the dyno. The car passed at 15mph and failed the first test at 25 mph at 1500rpm. The retest was run at 2300rpm and the car squeaked by.

My conclusion (how ever wrong it maybe) is that at higher rpm's more air is forced into the cats, diluting the Nox? Although my car passing at a higher rpm could be nothing more then a coincidence, I thought I'd throw it out.

Best of luck.

ViribusUnits

I've noticed that as well in other cars, as have other rennlisters in 928s. I have a diffrent theory as to why, but no "proof" as to if I'm correct or not.

Qustion, do you have an automatic transmition?

ROG100

mines an auto

Greggles

ViribusUnits
what is your theroy, maybe my experience can further support it?

John Speake

ROG100,
I haven't had time to follow all this thread, but I'm sorry to hear you still have a problem.

While the car is operating "steady state" rpm, then the O2 loop should keep the mixture at stoich.

If the MAF was way out of cal, then you would get a fault code. Did the Porsche or other garage put a Hammer on it ?

ViribusUnits

My theory:

For a given power output, the amount of air flowed through the engine is about the same. Not exactly, but close enough for this theory to fuction.

Thus, when you run it at a higher rpm on the dyno, less air is taken in per revolution.

Less air in the cylinder per revolution means lower pressures, and lower peak temps. Therefor lower NOx.

Here is what I can confirm:

1. Running the engine at a specific power out put, but higher revolutions results in more manafold vacuum. To measure, I put a vacuum guage on my car, and then ran at 55 mph in 5th gear, and in 3rd gear. Perdictably, 3rd gear had MUCH more vacuum.

This logicaly means the second point is true. And that would tend to state that the 3rd is true, BUT I can't draw a corolation without putting a bunch of cars on the dyno, and proveing it, which I don't have the time or cash to do. That or putting temp. and pressure sensors in the combustion chamber, and proveing it that way. Again, no time or cash.